The first recorded dust explosion occurred in a flour mill in 1785. Why then, 200 plus years later, are we continuing to have dust explosions and kill people?
- 2022, India, sugar dust explosion due to a short circuit at a conveyor belt killed two workers and injured nine.
- 2017 USA, Didion Milling, multiple corn dust explosions killed 5 workers and injured 14.
- 2015, UK, wood dust explosion resulted in the deaths of four workers and serious injuries to several others.
All these accidents and fatalities could have been avoided if only the correct dust explosion hazard strategy had been properly implemented. This articles outlines how this can be achieved by looking at the conditions needed for a dust explosion to occur and the options for safety.
How Dust Explosions occur?
We eat food because they are full of energy. So foodstuffs like flour, sugar, corn can burn – in fact it’s best to assume that all such organic powders are combustible and can burn.
Food products account for about 50% of dust explosions, with wood products making up 25% of dust explosions, and metals approximately 10%.
Metals in their natural state exist as an ore but, when extracted and refined to their pure element, are keen to react with oxygen. In 2014 75 people were killed in a dust explosion at a facility where aluminium car wheels were polished.
A fire triangle is a common expression and is a good visualisation tool to aid understanding that fuel, air and ignition is required for a fire to occur.
In the UK, fire safety is generally based on adequate means of detection and escape. That is because we can retreat to a place of safety before the effects of a fire become harmful.
However, should a flammable dust cloud of combustible concentration occur in the open then, on ignition, a flash fire can occur.
A 1mm dust layer with a bulk density of 500kg/m3, when roused to give a concentration of 100 g/m3, will form a flammable atmosphere up to a height of 5m. These figures are typical of foodstuffs like flour. When the dust cloud is ignited it will rapidly expand in volume by (say) a factor of ten, thus engulfing anything nearby.
When the flammable dust cloud is confined then we have the explosion pentagon, and a devasting explosion can occur.
Dust explosions have typically a maximum explosion pressure of 7 to 10 barg. They can therefore destroy buildings and cause fatalities. To manufacture powder based products they have to handled and processed using equipment such as pneumatic and mechanical conveyors, mills, blenders, screens and dust filters.
Powder handling equipment provides many potential ignition sources such as hot bearings, slipping belts, smouldering powders, electrical equipment. In addition, all powder movement generates electrostatic charges. Modern day powder handling plants therefore present many opportunities for creating the Dust Explosion Pentagon.
Why are Dust Explosions so Devastating?
Food products account for about 50% of dust explosions, with wood products making up 25% of dust explosions, and metals approximately 10%
Invariably, when dust explosions result in multiple fatalities and the destruction of buildings, it is caused by a secondary Dust Explosion. When a small primary dust explosion occurs a pressure wave is created and, as that travels, surrounding dust layers are roused.
These may be dust layers on the floor, on equipment, and on high ledges. Immediately following the pressure wave is a flame front ready to ignite the explosive dust cloud that has just been created by the travelling pressure wave.
Figure 4: Secondary Dust Explosion
When the Imperial Sugar dust explosion occurred in 2008, which killed 14 people, secondary dust explosions could be heard going off for 15 minutes afterwards.
One of the most important control measures in managing the explosion risk is keeping the process buildings in a clean condition. This entails containing dust and / or capturing vented dust using a Local Exhaust Ventilation (LEV) system.
Where dust deposits do form, then housekeeping must prevent dangerous situations occurring. The standard for good housekeeping is very demanding, with BS EN 60079-10-2[1] providing the following definitions:
Good: Dust layers are kept to negligible thickness, or are non-existent. In this case, the risk of the occurrence of explosive dust clouds from layers and the risk of fire due to layers has been removed.
Fair: Dust layers are not negligible but are short-lived (typically less than one shift). The dust is removed before any fire can start.
Poor: Dust layers are not negligible and persist for a long period of time (typically more than one shift). The fire risk and secondary explosion risk may be significant.
It follows that the site’s housekeeping protocol must prevent dust layers that are not negligible from forming. As a guide, if dust layers are present such that you could see footprints, then they need to be removed.
Achieving fair to good levels of housekeeping is very challenging for some industries, but must be achieved.
How do I know if my Plant is Safe to Operate?
When dust explosions result in multiple fatalities and the destruction of buildings, it is often caused by a secondary dust explosion
Do you have the documentation so that you can provide the necessary evidence required to demonstrate that your process is safe to operate?
To know that your plant that is handling and processing combustible powders is safe, then you need to have in place, for each unit operation, a suitable Basis of Safety.
As, unlike with a fire, a person cannot outrun an explosion, then safety is placed on prevention or protection. In the hierarchy of risk decision making, prevention is preferable to protection.
To base safety on prevention, the fire triangle must be broken. The Basis of Safety for Explosion Prevention is therefore
- the avoidance of a flammable atmosphere (either by removing the fuel of removing the air / oxidant), or
- the elimination of ignition sources.
Where the measures put in place to avoid an explosion cannot be achieved with a satisfactory level of certainty, then it is recognised that there is the possibility of an explosion. The effects of the explosion must be mitigated and therefore the Basis of Safety is Explosion Protection. This is achieved by
- Containment,
- Suppression, or
- Relief.
It is critical when providing explosion protection that the explosion is not allowed to travel to upstream or downstream connected equipment. If the explosion is allowed to propagate, then explosion piling could occur.
Pressure piling occurs when the travelling pressure wave pre-pressurises adjoining vessels before the flame front reaches them, thus resulting in exacerbating the explosion. Thus where explosion protection is fitted, explosion isolation is required.
There are several means of achieving explosion isolation, e.g. fast shutting gate valves, explosion flaps, isolation valves, chemical barriers, rotary valves.
Conclusion: How Safe is Safe Enough to prevent Dust Explosion?
Author:
Paolo Manfredini is a Process Safety Consultant specialised in DSEAR/ATEX and leading PHA studies. He has worked in both the pharmaceutical and agrochemical industries. He can be contacted via email at paolo.manfredini@dekra.com or on 07483143202.
In the UK, it is for you to demonstrate your plant is safe enough to operate. The legal[2] requirement where risk cannot be eliminated, so far as is reasonably practicable, is to apply measures to control risks and mitigate any detrimental effects.
For powder handling plants there is no prescriptive rule book on how this is to be achieved. Fortunately there is plenty of useful guidance and a good place to start would be the HSE website and their guidance book HSG103, Safe handling of combustible dusts: Precautions against explosions.
Ultimately it is down to the user of the powder handling plant to show the necessary documentation and risk assessments that the powder processing plant will not explode.
[1] BS EN 60079-10-2:2015 Explosive atmospheres. Classification of areas. Explosive dust atmospheres
[2] The Dangerous Substances and Explosive Atmospheres Regulations 2002
